Often defined as any unwanted sound that is not related to the wanted sound (if it is related we call it distortion). In electronics it can be further defined as a wide band addition to a signal by any electronic or mechanical component. "Random noise" is the most common type. It is unpredictable and contains a continuous distribution of energy over all frequencies - or at least all frequencies relevant to the system at hand. There are other types of noise, most of which are artificially created and have specific uses.

Noise is present in everything. Virtually any device carrying a signal adds noise. This includes, wire, speakers, amplifiers, and so on. Some devices add much more than others. Consequently an important measure of the quality of a signal is its signal to noise ratio, which describes how powerful the signal is in relation to the noise accompanying it.